1. Technical Field
The present invention relates to an alignment measuring system in a photolithography process that is capable of enhancing a measurement precision for an alignment mark on a wafer.
2. Description
A photolithography process is provided to prescribe respective different pattern images that a plurality of reticles has on a wafer. The pattern images are sequentially transferred onto the wafer, to thus become a required circuit pattern through a process such as etching, film deposition, etc.
In this photolithography process, it is important that a design of a precise circuit pattern and each different pattern layer constituting a circuit pattern are precisely mutually aligned and overlaid.
At present, in managing the overlay there are many efforts to realize a more integrated and precise circuit pattern by revising the reticle pattern, changing the photoresist, etc.
Herewith, the size of the pattern is almost determined by the specifications of the equipment and the photoresist, but an improvement for the overlay of each pattern image is being successively required as a regular preventive maintenance or a development of a measuring system.
An ultimate purpose of overlay management is to exactly overlay the pattern layer of the transferred pattern image with an existing pattern layer(s), and to provide data for a continuous execution of a process involving developing or correcting misalignment of the existing pattern, and to provide a standard to determine whether or not rework should be performed through the measurement of the overlay.
Thus, in order to precisely align the pattern layers through exact detection data for an alignment state of a wafer and a reticle, it is required to exactly detect a position of an alignment mark that indicates an alignment state of the wafer.
A system for measuring a position of an alignment mark in aligning a wafer will be described as follows, referring to FIG. 1.
According to the prior art, with reference to FIG. 1, the alignment measuring system includes a light source 14, first and second splitters 16 and 20, a reference mirror 22, a focusing diode 18, and an image sensor 12.
The alignment measuring system operates with a stage 10 on which a wafer is positioned. The stage 10 is adapted to rotate by a given angle in a horizontal (X-axis) direction, and a vertical (Y-axis) direction, and to be ascendible and descendible with a height control in each direction, in response to a control signal of a controller (not shown).
The image sensor 12 is disposed above the stage 10, opposite and confronting the top surface of the stage 10 on which a wafer is mounted. The first and second splitters 16 and 20 are provided along a straight line between the stage 10 and the image sensor 12. The light source 14 is disposed at one side of the first splitter 16, and the reference mirror 22 is disposed at the other side. The focusing diode 18 is disposed along one side of the second splitter 20.
The alignment measuring system of FIG. 1 operates as follows. The light source 14 produces light and directs the light toward the first splitter 16. The first splitter 16 is adapted to direct a first part of the light emitted from the light source 14 toward the wafer on the stage 10, and a second part of the light emitted from the light source 14 toward the reference mirror part 22. The wafer receives the light from the splitter 16, and reflects and/or diffracts at least a portion of that light back toward the first splitter 16. The first splitter 16 passes the reflected and/or diffracted light from the wafer toward the second splitter 20. The second splitter 20 is adapted to direct a first portion of the reflected/diffracted light from the wafer toward the image sensor 12, and a second portion of the light toward the focusing diode 18. The image sensor 12 is adapted to detect light reflected vertically from the wafer, or diffracted therefrom.
The controller combines information provided through the reference mirror part 22, the focusing diode 18, and the image sensor 12 to detect a focus for a positional state of the wafer.
Subsequently, in a procedure of scanning an upper face of a wafer, the diffracted light from the wafer, namely, a light signal diffracted by an alignment mark on the wafer, is detected to check an aligned state of the wafer, thus the alignment position of the wafer is determined to control the alignment of the wafer on the stage 10.
Herewith, in the path of the light from the light source 14 to the image sensor 12, the light is dispersed toward the wafer and the reference mirror part 22 through the first splitter 16, and the light reflected from a surface of the wafer also becomes dispersed light toward the first splitter 16. The reflected light transmitted through the first splitter 16 is also dispersed toward the focusing diode 18 and the image sensor 12 through the second splitter 20.
Accordingly, the light level reaching the image sensor 12 is reduced through the many stages of dispersed light, as compared with the light first emitted by the light source 14, and this lowers the reliability of the detection of alignment position of the wafer.
An error of the alignment position causes an abnormal transfer of the pattern image, to bring about a great deal of process defects and an increased rework rate for the process, and degrades the working efficiency and productivity.
Accordingly, it would be desirable to provide an alignment measuring system in a photolithography process to increase reliability in detecting an alignment position of a wafer and prevent a process defect, and to increase the productivity and working efficiency and enhance the product quality.
To achieve these objects, an alignment measuring system of a photolithography process includes a focusing diode, a light source, an image sensor, first and second splitters, and a controller. The light source is adapted to emit light. The first splitter is adapted to direct a portion of the light emitted from the light source toward a wafer disposed on a stage, and to direct in a first direction a part of reflected light from the wafer. The second splitter is adapted to receive the part of the reflected light from the first splitter, to direct a first portion of the received light toward the image sensor, to direct a second portion of the received light toward the focusing diode, and to control levels of the first and second portions of the received light in response to an applied control signal. The image sensor is adapted to receive the first portion of the light from the second splitter and to produce a detection signal therefrom. The controller is adapted to receive the detection signal from the image sensor to determine an alignment state of the wafer, to control the stage so as to align and position the wafer, and to apply the control signal to the second splitter.